batman-adv: add bat_neigh_is_equiv_or_better API function

Each routing protocol has its own metric semantic and
therefore is the protocol itself the only component able to
compare two metrics to check their "similarity".

This new API allows each routing protocol to implement its
own logic and make the external code protocol agnostic.

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>